Painful diabetic neuropathy (PDN) is a common and debilitating peripheral nerve complication of diabetes mellitus. By 1 year after the initial diagnosis of diabetes, 7% of patients report symptoms (e.g. pain, abnormal sensations) with the prevalence rising to 50% by 25 years of diabetes diagnosis (Sima and Sugimoto, Diabetologia 42: 773-788 1999; Cameron et al. Diabetologia 44: 1973-88 2001). Patients may present with one or more symptoms including burning sensations, lancinating and deep aching pains, depending upon the extent of nerve injury (Boulton, Diabetes Metab Res Rev 19: S16-21 2003). Numbness, tingling, and a sensation of tightness in the extremity are also commonly associated with PDN (Boulton, Diabetes Metab Res Rev 19: S16-21 2003). In addition, unpleasant abnormal sensations (dysaethesia), enhanced sensitivity to stimulation (hyperaesthesia), a heightened response to painful stimuli (hyperalgesia) and a distorted sense of touch producing allodynia (innocuous stimuli such as light brushstrokes of the skin produce pain) are all commonly reported by patients with diabetic neuropathy (Boulton, Diabetes Metab Res Rev 19: S16-21 2003).
There are no preventative treatments for PDN (Sima et al. Diabetologica 42: 773-788 1999), hence the therapeutic management of the condition is primarily palliative. This palliative management also represents a significant therapeutic obstacle, as the most efficient analgesic pharmaceuticals available, the μ-opioid receptor agonists such as morphine, are reportedly ineffective for the relief of PDN (Attal, Clin J Pain 16: S1118-30 2006). The mechanism underpinning the development of this opioid agonist hyposensitivity is unclear, but investigations have shown that poor glycaemic control can reduce pain tolerance and pain threshold and thus reduce the effectiveness of analgesics such as morphine (Morley et al. Am J Med 77(1): 79-83 1984). In addition, there may be diabetes-associated alterations in morphine pharmacokinetics (Courteix et al. J Pharmacol Exp Ther 285(1): 63-70 1998) and/or changes in opioid receptor function (Chen et al. Anesthesiology 97: 1602-1608 2002).
Although PDN is attributed primarily to poor glycaemic control over a prolonged period, the exact pathogenesis is poorly understood (Sima and Sugimoto, Diabetologia 42: 773-788 1999; Feldman et al. Curr Opin Neurol 12: 553-63 1999) Presently, there are two broad theories regarding the development of PDN: the vascular dysfunction theory and the metabolic dysfunction theory.
The vascular dysfunction theory proposes that changes in the blood supply to the nerves (the neurovasculature or vasa nervorum) occur secondary to haemodynamic abnormalities (such as accelerated platelet aggregation and increased blood viscosity) (Fusman et al Acta Diabetol 38(3): 129-34 2001). In addition, pathological changes in the small blood vessels of the neurovasculature may occur (such as reduction of the production of nitric oxide from the endothelial cells of blood vessels and acceleration of the reactivity on vasoconstrictive substances) (McAuley et al. Clin Sci (Lond) 99(3): 175-9 2000). These haemodynamic and vascular changes, acting independently or synergistically, are capable of causing the perineurial ischemia and subsequent endoneurial hypoxia observed in human patients and animal models of diabetes (Cameron et al. Diabetologia 44(11): 1973-88 2001). The end result of these abnormalities is nerve damage capable of causing the symptoms and signs of PDN.
On the other hand, in the metabolic dysfunction theory, the causes of nerve damage are mediated through the activation of the polyol metabolic pathway and through non-enzymatic protein glycation. These pathways induce mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiencies in the nerves which can culminate in damage to neural and neurovascular tissues (Obrosova et al. FASEB J 16(1): 123-5 2002). In addition, these metabolic changes are thought to activate protein kinase C (PKC) which is capable of heightening pain responses (Kamei et al. Expert Opin Investig Drugs 10(9): 1653-64 2001) and also of producing μ-opioid agonist hyposensitivity (Wang et al. Brain Res 723(1-2): 61-9 1996). Furthermore, heightened PKC activity is thought to reduce the binding affinity of μ-opioid receptors for ligands (Ohsawa et al. Brain Res 764:244-8 1998). The consequences of these metabolic abnormalities are nerve damage and the development of μ-opioid agonist hyposensitivity, as seen in patients with PDN.
It is likely that neither theory is mutually exclusive and proponents of both theories converge in the belief that, downstream of vascular dysfunction or metabolic abnormalities, there is an imbalance in the production of vaso-active compounds in the vasa nervorum which leads to hypoxic ischemia of diabetic nerves.
Of all the endogenous vasodilators, nitric oxide is the most potent and hence is a likely candidate for reduced synthesis and consequent diabetes-induced constrictions in vascular tone. As well as relaxing vascular smooth muscle, it also inhibits the processes of platelet aggregation, mitogenesis and proliferation of cultured vascular smooth muscle, and leucocyte adherence (Wroblewski et al Prev Cardiol 3(4): 172-177 2000). Nitric oxide is produced by the vascular endothelium by a group of enzymes called nitric oxide synthases. There are three isoforms of nitric oxide synthase (NOS) named according to their activity or the tissue type in which they were first described. These enzymes all convert the endogenous substrate, L-arginine, into L-citrulline, producing NO in the process.
Recent studies by the present inventors revealed unexpectedly that nitric oxide donors such as L-arginine can broadly prevent, attenuate and/or reverse the development of reduced analgesic sensitivity to an opioid receptor agonist such as morphine in neuropathic conditions, including peripheral neuropathic conditions such as PDN (see International Publication No. WO 2003/078437). This finding that nitric oxide donors can restore the analgesic sensitivity of opioid analgesics such as morphine in subjects with neuropathic conditions was significant because it allowed the use of these analgesics for treating or preventing pain in conditions, for which they were previously considered ineffective.